You need to firstly decide what you want it for – are you going to be racing or just going out for leisure rides and maybe doing the odd sportive? This will determine the geometry of the bike.
Bibshorts, shoes and pedals could set you back anything from £100 upwards. You also may need a new cycling top with rear pockets which is another £30 at least, and possibly other bits and pieces (small saddle bag, mini pump, spare tubes etc). So you may be looking at a bike around the £800-£850 price points.
Don't discount the larger brands such as Specialized, Trek, Giant and Bianchi where you will get a decent aluminium frame with carbon forks and either Tiagra or 105 kit. Other brands such as Cube, Felt and Cannondale are also worth a look. The crucial thing is to try before you buy – fit is much more important on a road bike than on a mtb.
Lots of threads on this exact question over on Bike Radar forums.